The Vacu Vin Wine Saver is one of my top-five favorite kitchen items, EASY. It's a small rubber stopper and a plastic pump that you use to remove excess air from a partially-consumed bottle of wine. With most of the air removed, the oxidation process that spoils wine is slowed. It makes leftover wine not only drinkable and delicious the next day, it keeps it pretty nice for even two or three days after, in my experience.
I can be a picky wine drinker and wine that was opened the day before is usually not my favorite. It definitely tastes...off to me. But since I have a Vacuvin I can open a bottle (even a nice bottle) guilt-free, if I know we're not going to drink it all that same night. It won't go bad, I won't have to pour it out, and I can have some more tomorrow.
We've had ours for several years and have had zero problems. It lives up the the expectation, it's simple to use, it's dishwasher safe, and it's less than $15.
(Photos from: Amazon.com)